Would I have to form an LLC to do freelance logo design and web design
and development through sites like Guru and Elance?
Legally, you do not. As you know, the purpose of the LLC is to protect your personal assets from liability, not to qualify you to do business.
This business doesn't sound very risky so could I get away with
forming a sole proprietorship?
None of us can make that subjective judgment for you given the current amount of information, especially if it constitutes legal advice. However, from my experience I can say most lawsuits in the field you're working in arise from intellectual property disputes (e.g. trademark and copyright infringement) so think about whether you'd be stepping on anyone's toes in that regard. One example of a possible suit is if your design outcome is similar to that of someone else's and they sue you for trademark infringement. Another example could involve a dispute over the ownership of the design you created for your client, especially if s/he pays for some of your services but suddenly aborts payment.